Graphics Channels

In the Advanced menu, there's a submenu called "Graphic Channels", where you can see what channels are active, and toggle them individually. There are 15 channels to choose from, where channel 1 is the default. Easier than that, you can just use the numeric keypad, 1-9 and shift 0-5 (shift adds 10 to the number you press).
For example, on some discs, there are multiple languages in a single song. English may be on channel 1 and French on channel 2. To switch from English to French, just press the 2 on the numeric keypad.